A thin film containing a metal oxide such as tin oxide has the function of reflecting infrared rays. Since a glass sheet provided with this thin film reduces the total solar energy transmittance and does not allow the heat within rooms to escape to the outdoors, it is widely available in the market as a low-emissivity glass. This thin film also can exhibit the function of shielding electromagnetic waves. A known method for manufacturing a glass sheet of this kind is such that a thin film of a metallic compound is formed on a high temperature glass surface utilizing thermal decomposition methods, such as a chemical vapor deposition method (CVD method) and a spraying method in which a solution material or a solid material is sprayed. For example, JP 11(1999)-509895 A describes a method of forming a thin film of a tin oxide by supplying a gaseous reaction mixture containing an organic tin compound, hydrogen fluoride, oxygen, and water onto a high temperature glass surface. JP 6(1994)-47482 A describes a method of forming a thin film of tin oxide by supplying a vapor of an organic tin compound on a glass ribbon surface in a float bath in a float manufacturing process. The use of the organic tin compounds such as described in these patent publications as a raw material for a thin film has an advantage that the thickness of the thin film easily is made uniform. Nevertheless, because organic tin compounds have high environmental loads as with tributyltin compounds, the use of alternative raw materials that replace organic tin compounds has been desired in recent years. Meanwhile, tin chloride conventionally has been used widely as a raw material for a tin oxide thin film in thermal decomposition methods. For example, JP 2(1990)-175631 A describes a method of forming a coating film in which, with a CVD method, a first flow of tin tetrachloride and a second flow of water vapor are supplied onto a glass in a turbulent flow state. Also, JP 9(1997)-40442 A describes a chemical vapor deposition method of forming a tin oxide on a glass of a substrate by causing tin tetrachloride and water to react with each other, wherein tin tetrachloride and water are supplied by separate flows at temperatures in a range of 100° C. to 240° C. and a single flow is formed and directed to the substrate substantially with a laminar flow to cause them to react with each other in a substrate region, whereby the glass is coated.
Such methods of forming a thin film containing a metal oxide that utilize thermal decomposition methods are inferior to physical vapor deposition methods, such as a sputtering method, in that it is difficult to obtain a uniform film thickness; nevertheless, they are capable of forming a thin film over a wide area within a short time at a relatively uniform thickness and therefore are suitable for mass production of industrial products. With the thermal decomposition methods, generally, the higher the temperature of the reaction system is, the faster the film deposition rate, although the situations vary somewhat depending on the compositional components of the raw materials for the thin film or the like. Accordingly, it seems that higher temperatures are preferable for the formation of a thin film in industrial production processes.
In the above-described thermal decomposition methods, the use of alternative raw materials has been desired because organic tin compounds have large environmental loads. The present inventors carried out a further experiment on the method described in JP 2(1990)-175631 A, which uses tin tetrachloride, an inorganic tin compound, and as a result found that when a first flow of tin tetrachloride and a second flow of water vapor are separately supplied to react with each other on a substrate, or when a reaction gas stream is supplied with a turbulent flow, non-uniformity in film thickness is caused in the formed thin film. The present inventors also performed a further experiment on the method described in JP 9(1997)-40442 A, in which tin tetrachloride and water are mixed and supplied onto a glass substrate with a laminar flow. It was confirmed that it was true that performing a coating according to this method eliminates non-uniformity in film thickness. However, due to high reactivity of tin tetrachloride with water, even when a pipe or the like for the mixed gas was controlled at a temperature of 100° C. to 240° C. as described in JP 9(1997)-40442 A, the pipe was clogged up by the reactant that deposited inside the pipe after only about 3 hour's supplying of a mixed gas of tin tetrachloride and water, making further gas supply impossible. Then, hydrogen chloride was further added to the mixed gas of tin tetrachloride and water vapor, and the mixed gas was supplied, as described likewise in Example D of JP 9(1997)-40442 A. In this case, hydrogen chloride acted as a reaction inhibitor, so the pipe was not clogged up in a short time. Nevertheless, although the clogging of pipe did not occur when hydrogen chloride and tin tetrachloride were supplied at a mole ratio of 1:1 as in Example D of JP 9(1997)-40442 A, the film deposition rate considerably lowered to about ⅓ of that in the case in which hydrogen chloride is not mixed because hydrogen chloride served as a reaction inhibitor.